This invention relates generally to thermometry, and more particularly, to improvements in a device commonly known as a diode thermometer, and circuitry associated therewith.
Electrical circuit components typically exhibit electrical characteristics which are temperature related. Hence, it is possible to use an electrical or electronic circuit component as a temperature sensor and to extract a measurement of temperature from the component by monitoring an electrical characteristic, provided that a correlation of the electrical characteristic with temperature is known. An example of a device of this type is commonly known as a diode thermometer.
In a solid state diode, semiconductor materials are arranged to provide an electrical characteristic which exhibits very high forward conductivity and extremely low reverse conductivity. In the conventional diode thermometer which operates at constant current, the voltage developed across the diode is related to temperature. Conventional diode thermometers have several disadvantages:
(1) the diode voltage is not precisely linearly related to temperature even for an ideal diode;
(2) at the same current and temperature, different diodes develop different voltages to the extent that diodes used in conventional diode thermometers must be individually calibrated, variations of .+-.50 degrees are typical; and
(3) the electrical properties and hence calibration of conventional diode thermometers change with time and thermal history.
One known temperature transducer arrangement which overcomes some of the problems in the prior art is described in U.S. Pat. No. 3,430,077 which issued to Bargen. This reference shows a semiconductor device having more than one junction and which is energized by a current having an alternating characteristic. In this manner, the effects of leakage currents and recombination currents are reduced. Also, if the junction is operated forward biased, with a forward voltage which exceeds approximately 0.1 volt, the change in voltage resulting from the various current levels is mathematically directly proportional to the absolute temperature.
U.S. Pat. No. 4,165,642 describes a monolithic integrated CMOS circuit which provides at its output a digital signal responsive to temperature. This known temperature sensing arrangement is of the bipolar bandgap reference type in that it requires two matched bipolar transistors. Such transistors must be matched in terms of areas of the junctions, doping densities, doping profiles, aging effects, and transistor temperatures. Commercially available systems which utilize this known concept typically produce chip-to-chip variations which result in temperature measurement variations of .+-./-5.degree. C. There is clearly a need for a temperature measuring system having greater accuracy.
A system wherein a digital signal is scaled to correspond selectably to Celsius and Fahrenheit values is described in U.S. Pat. No. 4,370,070. In accordance with the teachings of this patent, the conversion from Farenheit to Celsius is achieved by dropping four of each nine counts to an accumulating counter. This is equivalent to multiplying by 5/9. Such counts are obtained from a temperature oscillator which generates a train of pulses at a frequency which is related to the sensed temperature. Also, the system described in this reference assumes linearity of the output of a thermistor with respect to temperature.
It is, therefore, an object of this invention to provide a simple and inexpensive temperature measuring arrangement.
It is another object of this invention to improve the accuracy with which temperature is measured.
It is also an object of this invention to provide a temperature measuring arrangement which can easily be produced as an integrated circuit.
It is a further object of this invention to provide a temperature sensing system which does not require matching of solid state devices at the sensor to achieve high accuracy.
It is additionally an object of this invention to provide an integrated circuit temperature sensing system which utilizes differential current values to achieve temperature measuring.
It is yet another object of this invention to provide a system wherein precisely accurate differential currents are generated.
It is a still further object of this invention to provide a temperature measuring arrangement wherein the effects of noise are substantially reduced.
It is an additional object of this invention to provide a CMOS integrated circuit wherein 1/f noise is substantially reduced.
It is still another object of this invention to produce a temperature measuring system which utilizes a down integrator having an extended dynamic range.
It is a yet further object of this invention to provide a temperature measuring system which achieves high accuracy and resolution without the need for calibration using water baths.
It is additionally a further object of this invention to provide a high accuracy temperature measuring system which does not require an amplifier having precisely predetermined gain.
The foregoing and other objects are achieved by this invention which provides a temperature sensing system having a solid state temperature sensor having a junction for developing a junction voltage thereacross. The junction voltage is responsive to a current which is conducted through the sensor, and is at least partially responsive to the temperature of the sensor. The current is generated by a current source, or generator, at first and second current levels so that the junction voltage varies correspondingly. A voltage value storage arrangement stores, in one embodiment, at least two junction voltage values. Such stored junction voltage values are received by a measurement arrangement which produces a signal responsive to the junction voltage values and the temperature of the sensor.
In one embodiment of the invention, the junction voltage values are stored in sample-and-hold devices which are controlled by a controller which may be a programmable system, such as a microcomputer. The values stored in the sample-and-hold devices are converted into a difference signal which is then integrated by an integrator. In a preferred embodiment, the integrator is of the type which integrates bidirectionally. The integration in one direction, illustratively a down integration, is timed. This results in a timing signal which corresponds accurately to the temperature of the solid state temperature sensor.
An auto-zero system is coupled to the integrator to assist in establishing a reference value for the integration. Additionally, this system functions to compensate for any offset voltage of the circuitry. Thus, if the junction difference voltage is zero, then the auto-zero circuit will cause the output of the integrator not to change with time.
In accordance with a further embodiment of the invention, a comparator is provided for receiving at respective inputs thereof signals from the integrator and the auto-zero system. The output of the comparator is conducted to the controller for indicating to the controller the duration of the timing interval. In addition, the controller is provided with means for scaling the number of clock pulses which are accumulated in a counter so as to provide counts which correspond to Fahrenheit and Celsius measurements.
The accuracy of the temperature measurements is improved in highly advantageous embodiments of the invention by providing a switching arrangement wherein minor matching errors in a plurality of current sources which provide the multilevel current are reduced by averaging them amongst themselves. This is achieved by providing n such current sources, each with an associated switch which may be controlled by the controller. When a high current level is to be conducted, all n switches are closed. When a low current is to be conducted, only one such switch is closed, thereby maintaining a current ratio of n:1 between the high and low levels. Preferably, various ones of the n switches are sequentially closed during conduction of the low level current to ensure that mismatches in the magnitude of the currents produced by the various current sources are averaged out.
In accordance with a method aspect of the invention, a temperature measurement is achieved by alternatingly conducting a current through a solid state sensor at first and second current levels; sampling and storing correspondingly produced voltages of the solid state sensor; producing a signal which corresponds to the difference between the correspondingly produced voltages; and integrating the difference signal.
In one embodiment, at least a portion of the period of time during which the integration occurs is measured to produce a timing signal which corresponds to the temperature of the sensor. In a specific illustrative embodiment of the invention such timing incorporates the steps of gating a clock signal to a counter, so as to count clock pulses, and preloading the counter with values which depend on whether the temperature is desired to be measured with Fahrenheit or Celsius scales.
In an integrated circuit embodiment of the invention, a temperature measurement system which can provide a readout in units as small as 0.1 degrees would require an operational amplifier having precisely predetermined gain. Such precision can be achieved, in accordance with the invention, by switching identical resistors so that all such resistors are used for the input and feedback loop during each measurement cycle. Alternatively, such switching can be used for capacitors. Thus, any mismatches are averaged out in a manner similar to that described hereinabove with respect to the current sources.
In accordance with a significant aspect of the invention, the disadvantageous effects of various types of noise are substantially reduced. There are essentially three types of noise which affect the precision to which the temperature can be measured. These include, synchronous noise, white noise, and 1/f noise. Synchronous noise results from pick-up from the clock and any circuit elements which are triggered thereby. To eliminate synchronous noise, it is essential that the clock and its associated synchronous circuitry pass through the various logic states as contemporaneously as possible. For example, the auto-zero phase should be identical to the measurement phase. Thus, the output of the differential amplifier which subtractively combines the outputs of the sample-and-hold devices will cancel out the synchronous noise.
White noise can be reduced to a relatively low level by sampling the voltage difference for a sufficiently long period of time. Thus, this type of noise must be taken into consideration in determining the maximum sampling speed when practicing the invention.
The third type of noise, 1/f, is more complicated than the other two and is particularly acute in CMOS integrated circuit embodiments of the invention. The contribution of 1/f noise does not depend on the duration of the sampling time interval, but rather on Q, the ratio of the time interval between the end and start of consecutive sample-and-hold time windows, to the total time interval beginning with the start of a first sample-and-hold window and the end of a second such window. In other words, 1/f noise depends upon the ratio of the time between two samples to the total sampling period including the time between the two samples. It is assumed, for purposes of this disclosure that the output of the sample-and-hold is the average value of the input voltage during the sampling window. It can be shown that the more fundamental quantity, the signal-to-noise ratio is proportional to the square of the quantity 1-Q for small Q values. Therefore, in order to keep the signal-to-noise ratio from being degraded by approximately 50%, it is necessary to keep Q to less than 0.25. It is therefore necessary that the sample-and-hold circuitry be designed such that Q is less than 0.25, and preferably as small as possible, because the contribution of 1/f noise to the differentially sampled voltage, which yields the measured temperature value, depends upon the amplitude of the 1/f noise and on Q. It should be noted that the amplitude of the 1/f noise is not controlled easily by the practitioner of the invention. However, for a given amplitude of 1/f noise, if reducing Q to near zero does not reduce the noise in the measurement of the temperature to a sufficiently small value to achieve the desired measuring precision, it is nevertheless possible to reduce the noise to an acceptable level by adding together several differential samples of the temperature. The noise decreases as the reciprocal of the square root of the number of samples of the temperature added together. For example, adding 100 samples reduces the 1/f noise by a factor of 10, relative to a single differential sample. Such averaging is easily achieved during up integration in a dual slope integrator. It may also be achieved digitally by averaging the final digitized temperatures. Such averaging will reduce almost all types of noise.
In accordance with a further aspect of the invention, a third sample-and-hold device is connected to the output of the differential amplifier for for storing the difference voltage while the first and second sample-and-hold devices store values for the next sample period. Thus, the efficiency of operation of the temperature measuring system is improved.
In an operational cycle of a specific illustrative embodiment of the invention, the first step is to determine the auto-zero value of the system, so as to correct for errors such as operational amplifier errors and the nonlinearities of various circuit components which cause a zero signal voltage to result in a nonzero value of the temperature. Such auto-zeroing occurs during a time period which is a significant fraction of an entire measurement cycle, illustratively 1/3 to 1/2 of the whole cycle. Before the first samples of the voltage are made by the sample-and-hold devices, the integrator, which for the sake of simplicity has been designed so as to accept only positive values, must be initialized. This is required because the output of the difference amplifier could be either positive or negative. Such initialization can be achieved by requiring the integrator to integrate upwardly on a positive voltage which is greater than all of the charge which could be deposited by any possible offset voltages developed in the system, and which is stable for at least one measurement cycle.
After the foregoing initialization, the lower of the two current levels is conducted through the sensor and the first and second sample-and-holds are strobed in their normal fashion so that Q is less than 0.25. During the auto-zero cycle the sensor current is not switched. The resulting difference value is then strobed into the third sample-and-hold, which is coupled to the integrator until the next operation of the first and second sample-and-hold devices. The cycle is continued for the remainder of the auto-zero cycle. For an analog auto-zero embodiment, the voltage applied to the positive input of the integrator is adjusted until the output of the integrator does not change with time, and the comparator is adjusted to switch states at this voltage. Also, the same voltage is maintained on the positive terminal of the integrator as well as the comparator during the temperature measurement period. For a digital auto-zero circuit, the final step in the auto-zero cycle is to down integrate on the reference voltage in the normal fashion. The number of clock pulses occurring during the down integration is stored for later use.
The next step in the cycle is to measure temperature. The low current level is conducted through the sensor and the first sample-and-hold is strobed. Subsequently, the high current is conducted and the second sample-and-hold is strobed after the switching transient has settled. The differential sample is then strobed into the third sample-and-hold, and it is coupled to the integrator. The cycle is repeated for the duration of the up integration. Since the first and second sample-and-holds will quickly charge to their final states, assuming that the temperature is not changing rapidly, the transient will not be important, and the time between samples can be made quite short.
In a still further embodiment, the dynamic range of the integrator is increased. The range of an up-down integrator is determined by the ratio of the maximum voltage thereacross to the size of the least significant bit. The prior art has encountered difficulty in building an integrator with a range greater than 1000, or with more than 10 bits resolution. In the present invention the range of the integrator is increased by integrating up and down several times in one cycle, and counting the total number of counts separately for the up and down integrations. The dynamic range is therefore increased severalfold.
The present invention, as noted herein, is directed to a new and unique solid state thermometer system wherein the measurement of sensed temperature is obtained essentially free of the undesired properties which plaque know systems. The present invention thereby provides a temperature signal which is indicative of the true temperature alone, essentially free of other influences which affect a diode's electrical characteristic, and in addition, that signal is linearly related to temperature. This offers significant advantages not only for the diode thermometer itself but also for its method of manufacture. Because a diode thermometer embodying principles of the present invention eliminates the effect of these other influences, it means that diodes do not have to be matched to the associated circuitry nor does there necessarily have to be an adjustment in the circuitry to compensate for a particular characteristic of a diode which is used with the circuitry. Also, circuitry to correct for the non-linearity of a conventional diode thermometer is unnecessary. Hence, reliability, accuracy, and simplification in manufacturing procedures are some of the important attributes of the invention.
The invention is also well suited for manufacture in integrated circuit form by using integrated circuit fabrication technology. This means that the invention can be packaged in a commercial form as an integrated circuit "chip", another very attractive attribute of the invention. The principles of the invention, however, may be applied to circuits which are in other than integrated circuit form. Thus, the present invention enhances a diode thermometer in a number of important ways, with particular reference to linearity, accuracy, reliability, packaging, and manufacturing.